Whoever occupies 10 Downing Street on May 6 will not be able to enjoy the sparkling Canaletto which hung in the first-floor parlour when it was the home of Sir Robert Walpole, the first prime minister, and the cleverest and arguably most corrupt man in England.

Walpole died in 1745, leaving stupendous debts. When his grandson inherited the title and the debts in 1751 he was dubbed "the most ruined young man in England".

He sold the Canaletto - a view of the Grand Canal in Venice, above - with a companion view of the doge's palace.

They probably began the British taste for Canaletto, and should have gone to the National Gallery or stayed in Downing Street, but the government dithered about buying them to start the national collection. They remained in an English family collection until they were sold to a French collector in the middle of the last century.

The canal view comes up for auction at Sotheby's this summer with a top estimate of £8m.